TiPON PARASITIC Fungi 387 



(More clear than any other are the posslbiHtics of forecasting for 

 the downy mildews.) The potato PJiytoptliora is quite likely to 



June, J 



experience of 1889 and 1897. 



'/ 



season may not be amiss. During 1887, while in Iowa, it was 

 recorded that from March to August there was only 8.32 inches 

 of rainfall, followed by nearly ten inches in September. A study 

 was made of the downy mildews and the following note was re- 

 corded in a bulletin of the Botanical Department of the Iowa 

 Agricultural College for the year : " These observed facts show 

 that with the Peronosporeae there is no doubt that the species are 

 best suited to a moist season. The members of the genus Pcrono- 

 spora have in no instance been so abundant during the last two 'dry 

 years as before the drought came. I'here was a decided decrease of 

 the mildews the past year until the September rains came, and after 

 that a few weeks of warm weather followed, during which seed- 

 lings of various kinds sprung up, and on these, in some instances, 

 •Peronosporac made their appearance. In general the mildews were 

 found in early spring and after this, through the long dry summer, 

 in limited quantities, upon plants growing in moist places along 



streams and edges of pools." 



The genus Cystopiis seems less influenced by drought, but as 

 a rule there was less of the species and the infested specimens 

 were those best situated for obtaining moisture. In all cases where 

 Peronosporeae flourished they w^ere upon succulent hosts, and 

 even with these there were probably less growth of parasites but 

 greater manifestation of disease due to lack of vitality in the 

 hosts. These instances, therefore, form no exception to the gen- 

 eral rule, that dry weather is not advantageous for the develop- 

 ment of the Peronosporeae. The apple rust and the black rot of 

 the grape come in the same category with the potato rot. 



With the twig blights, due to bacteria that spread most largely 

 in early summer from flower to flower by means of bees or to the 

 tender opening leaf buds the case Is different. This would seem 

 to be favored by a wet early spring as evidenced by the remarkable 

 outbreak in 1894, when, in May, 7.72 inches of rain fell, the third 

 largest monthly precipitation during the nine years. 



